The Gauntlet Rite is a ritual that all eighteen-year-olds participate in, held at Mikado Castle. A specialized gauntlet - a COMP with which to summon demons - is put on the person's arm. If it activates, they pass the rite and become Samurai.
Flynn, along with many other candidates, participate in the ritual at the beginning of the game, and five of them pass, all becoming Samurai.